This commit declares and type-checks the following clauses:
* limit().offset() chains on SelectStatement
* onConflict().doUpdate() chains on InsertStatement
We need to flesh out the other possibilities for onConflict() in
particular before all these changes get submitted as an upstream
PR.
This will allow us to cleanly extend these interfaces with
additional clauses understood by Postgres, which we can then chain
without having to redeclare the entire interface.
Wrap everything in a module instead of a namespace. I'm still not
entirely clear on what the distinction is but the forked typings
don't seem to get applied without this change.
